1. Field of the Invention
The invention relates to a fuel injection system having a booster unit for being used in an internal combustion engine, such as a diesel engine.
2. Description of the Related Art
A fuel injection system which uses a boost type common rail system in a diesel engine is generally known. In a fuel injection system of this type, fuel of a high flow rate and a high pressure is used as hydraulic fluid to move a booster piston. The booster piston is provided between a pressure chamber and a backpressure chamber in an injector. The booster piston is driven in accordance with a differential pressure occurring between the pressure chamber and the backpressure chamber when the fuel in the backpressure chamber is discharged. The fuel boosted by the booster piston is transferred to a needle valve mechanism in a nozzle portion of the injector.
A fuel injection system is described in a patent document (PCT National Publication No. 2002-539372), for example. This fuel injection system includes a needle valve drive unit which opens and closes a needle valve of the needle valve mechanism. The needle valve drive unit includes a pressurization chamber which introduces fuel fed from a common rail; an open/close valve capable of discharging the fuel in the pressurization chamber; and a pressure-receiving piston accommodated in the pressurization chamber. The needle valve drive unit opens the needle valve in conjunction with the discharge of the fuel preserved in the pressurization chamber.
In the fuel injection system described in the patent document, the booster piston is driven by the discharge of the fuel in the backpressure chamber of a booster unit. An injection operation (fuel injection) is executed in accordance with the discharge of the fuel in the pressurization chamber of the needle valve drive unit. The fuel discharged from the backpressure chamber of the booster unit is returned to a fuel tank through a fuel discharge passage provided for the booster unit. The fuel discharged from the pressurization chamber of the needle valve drive unit is returned to the fuel tank through a return passage independent of the fuel discharge passage.
Thus, the conventional system requires the fuel discharge passage for the booster unit and the return passage for the needle valve drive unit. These passages each extend to the fuel tank. Therefore, two systems of pipings are necessary, therefore making a complex piping configuration.
To make a simplified piping configuration, attempts have been made to combine the fuel discharge passage for the booster unit and the return passage for the needle valve drive unit together in a portion close to the injector. However, the flow rate being discharged from the backpressure chamber of the booster unit is greater than the flow rate being discharged from the pressurization chamber of the needle valve drive unit. Therefore, if the fuel discharge passage for the booster unit and return passage for the needle valve drive unit are combined together, a mutual interference occurs between a discharge flow stream from a discharge valve of the booster unit and a discharge flow stream from the open/close valve of the needle valve drive unit, whereby normal discharge is not done. It was discovered that this deteriorates injection characteristics of the injector.